Effect of mirror system and scanner bed of a flatbed scanner on lateral response artefact in radiochromic film dosimetry.

Abstract Radiochromic film is a good dosimeter choice for patient QA for complex treatment techniques because of its near tissue equivalency, very high spatial resolution and established method of use. Epson scanners are widely used for film dosimetry. Previous studies reported that different components in the construction of flatbed scanners introduce light polarisation which is responsible for enhancing the Lateral Response Artefact (LRA) effect. The purpose of this study is to investigate the contribution of the scanner bed and mirror system of EPSON flatbed scanners to this effect. EBT3 films were irradiated with 40x40cm2 field size in 6 MV beams. Films were analysed using images captured by a Canon 7D camera utilising a 50mm focal length lens to evaluate the polarization introduced by various combinations of mirrors making up the mirror system. An EPSON V700 scanner is used to investigate the effect of the scanner bed. The magnitude of polarization introduced by the mirror system is found to be up to 14%, whereas the path length effect, from the difference in refractive index of film and scanner bed, is found to be insignificant (at less than 2%).